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<article class='withtoc'>
    <h1>
      监听下载事件
      <a class="edit-page-link" href="https://github.com/liaohuqiu/fresco-docs-cn/blob/docs/docs/01-listening-download-events.md" target="_blank">编辑和纠错</a>
    </h1>
    <p></p>

    <p>你也许想在图片下载完成或者下载失败之后，做一些其他事情。</p>

<p>图片是后台线程异步加载的，我们可以使用一个<code>ControllerListener</code>实现事件的监听。</p>

<p><em>在监听事件回调时，无法修改图片，如果需要修改图片，可使用<a href="modifying-image.html">后处理器(Postprocessor)</a></em></p>

<h4>使用方法</h4>

<p>简单定义一个<code>ControllerListener</code>即可，推荐使用<code>BaseControllerListener</code>:</p>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">ControllerListener</span> <span class="n">controllerListener</span> <span class="o">=</span> <span class="k">new</span> <span class="n">BaseControllerListener</span><span class="o">&lt;</span><span class="n">ImageInfo</span><span class="o">&gt;()</span> <span class="o">{</span>
    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">onFinalImageSet</span><span class="o">(</span>
        <span class="n">String</span> <span class="n">id</span><span class="o">,</span>
        <span class="nd">@Nullable</span> <span class="n">ImageInfo</span> <span class="n">imageInfo</span><span class="o">,</span>
        <span class="nd">@Nullable</span> <span class="n">Animatable</span> <span class="n">anim</span><span class="o">)</span> <span class="o">{</span>
      <span class="k">if</span> <span class="o">(</span><span class="n">imageInfo</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
        <span class="k">return</span><span class="o">;</span>
      <span class="o">}</span>
      <span class="n">QualityInfo</span> <span class="n">qualityInfo</span> <span class="o">=</span> <span class="n">imageInfo</span><span class="o">.</span><span class="na">getQualityInfo</span><span class="o">();</span>
      <span class="n">FLog</span><span class="o">.</span><span class="na">d</span><span class="o">(</span><span class="s">&quot;Final image received! &quot;</span> <span class="o">+</span> 
          <span class="s">&quot;Size %d x %d&quot;</span><span class="o">,</span>
          <span class="s">&quot;Quality level %d, good enough: %s, full quality: %s&quot;</span><span class="o">,</span>
          <span class="n">imageInfo</span><span class="o">.</span><span class="na">getWidth</span><span class="o">(),</span>
          <span class="n">imageInfo</span><span class="o">.</span><span class="na">getHeight</span><span class="o">(),</span>
          <span class="n">qualityInfo</span><span class="o">.</span><span class="na">getQuality</span><span class="o">(),</span>
          <span class="n">qualityInfo</span><span class="o">.</span><span class="na">isOfGoodEnoughQuality</span><span class="o">(),</span>
          <span class="n">qualityInfo</span><span class="o">.</span><span class="na">isOfFullQuality</span><span class="o">());</span>
    <span class="o">}</span>

    <span class="nd">@Override</span> 
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">onIntermediateImageSet</span><span class="o">(</span><span class="n">String</span> <span class="n">id</span><span class="o">,</span> <span class="nd">@Nullable</span> <span class="n">ImageInfo</span> <span class="n">imageInfo</span><span class="o">)</span> <span class="o">{</span>
      <span class="n">FLog</span><span class="o">.</span><span class="na">d</span><span class="o">(</span><span class="s">&quot;Intermediate image received&quot;</span><span class="o">);</span>
    <span class="o">}</span>

    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">onFailure</span><span class="o">(</span><span class="n">String</span> <span class="n">id</span><span class="o">,</span> <span class="n">Throwable</span> <span class="n">throwable</span><span class="o">)</span> <span class="o">{</span>
      <span class="n">FLog</span><span class="o">.</span><span class="na">e</span><span class="o">(</span><span class="n">getClass</span><span class="o">(),</span> <span class="n">throwable</span><span class="o">,</span> <span class="s">&quot;Error loading %s&quot;</span><span class="o">,</span> <span class="n">id</span><span class="o">)</span>
    <span class="o">}</span>
<span class="o">};</span>

<span class="n">Uri</span> <span class="n">uri</span><span class="o">;</span>
<span class="n">DraweeController</span> <span class="n">controller</span> <span class="o">=</span> <span class="n">Fresco</span><span class="o">.</span><span class="na">newControllerBuilder</span><span class="o">()</span>
    <span class="o">.</span><span class="na">setControllerListener</span><span class="o">(</span><span class="n">controllerListener</span><span class="o">)</span>
    <span class="o">.</span><span class="na">setUri</span><span class="o">(</span><span class="n">uri</span><span class="o">);</span>
    <span class="c1">// other setters</span>
    <span class="o">.</span><span class="na">build</span><span class="o">();</span>
<span class="n">mSimpleDraweeView</span><span class="o">.</span><span class="na">setController</span><span class="o">(</span><span class="n">controller</span><span class="o">);</span>
</code></pre></div>
<p>对所有的图片加载，<code>onFinalImageSet</code> 或者 <code>onFailure</code> 都会被触发。前者在成功时，后者在失败时。</p>

<p>如果允许呈现<a href="progressive-jpegs.html">渐进式JPEG</a>，同时图片也是渐进式图片，<code>onIntermediateImageSet</code>会在每个扫描被解码后回调。具体图片的那个扫描会被解码，参见<a href="progressive-jpegs.html">渐进式JPEG图</a></p>


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